Group III-nitride compound semiconductors are promising materials for use in making semiconductor light-emitting devices, such as semiconductor lasers and semiconductor light-emitting diodes. Such compound semiconductors are also promising for use in making such circuit devices as metal semiconductor field-effect transistors (MESFETs). In particular, semiconductor lasers based on such Group III-nitride compound semiconductors such as gallium nitride (GaN), aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN) or aluminum indium gallium nitride (AlInGaN) have great potential for generating light in the blue and violet wavelength ranges due to their substantially wider band gap than gallium arsenide, the material used to make lasers that emit red light. Lasers that generate light in the blue and violet are make it possible to increase the information storage density of optical storage devices. For example it is proposed to introduce digital versatile disks (DVDs) having a capacity of about 15 gigabits around the year 2000. A semiconductor material that includes at least gallium and nitrogen will be referred to in this disclosure as a gallium nitride semiconductor. A semiconductor that includes at least one Group III element and at least nitrogen will be called a Group III-nitride semiconductor. A semiconductor device that includes a Group III-nitride semiconductor will be called a Group III-nitride semiconductor device.
Although lasers based on gallium nitride semiconductors and capable of generating blue or violet light have been made, the lifetimes of current lasers based on gallium nitride semiconductors are extremely short at practical light emission intensities. Longer lifetimes are required. The major cause of the short lifetime of current devices is thermal degradation due to heat dissipation in the metal-to-semiconductor contacts through which current is fed through the device. The problem is particularly severe in the contact with a gallium nitride semiconductor doped with acceptor (p-type) impurities, i.e., the p-contact. The band gap of gallium nitride, at 3.4 eV, is comparatively high, which makes it difficult to make a low-resistance contact with this material.
FIG. 1 shows an example of an edge-emitting semiconductor laser 1 based on gallium nitride semiconductors. This device is formed by depositing the GaN buffer layer 3 on the sapphire substrate 2, and then successively depositing the n-type GaN contact layer 4, the n-type AlGaN cladding layer 6, the GaN waveguide layer 7, the InGaN multi-quantum well layer 8, the p-type GaN waveguide layer 9, the p-type AlGaN cladding layer 10, and the p-type GaN contact layer 11. The n-contact 13 is formed by dispositing the metal n electrode 5 on the n-type GaN contact layer 4 and the p-contact 14 is formed by depositing the metal p electrode 12 on the p-type GaN contact layer 11.
As will be described below, the contact resistance of the n-contact 13 is acceptably small. However, the contact resistance of the p-contact 14 between the p-type GaN contact layer 11 and the p electrode 12 is large. For example, in an typical device, the p electrode is formed from layers of gold and nickel and contacts an approximately 100 .mu.m.times.100 .mu.m area of the p-type GaN contact layer 11. Even if the specific contact resistance between the p electrode and the contact layer can be reduced to as little as 2.times.10.sup.-1 .OMEGA.cm.sup.2, the p-contact has a contact resistance of 2,000 .OMEGA.. Consequently, a drive current of 100 mA will dissipate about 20 W of power at the p-contact.
In 671 NIKKEI ELECTRONICS, p. 9, Nikkei McGraw-Hill Co. (Sep. 23, 1996) is described a practical gallium nitride semiconductor-based laser that generates blue light at a wavelength of 417 nm and that has a light-emitting layer with an InGaN multi-quantum well layer structure having 25 quantum wells. The drive voltage of this device is 20 V and its drive current is 5 A, corresponding to a continuous power dissipation of 100 W. Consequently, the duty cycle of the light emitted by this laser is limited to 0.001 at room temperature, corresponding to an average power dissipation of 100 mW. This laser cannot generate light continuously, and therefore cannot be used in applications in which continuous light emission is required.
In other applications, the high contact resistance of the p-contact increases parasitic resistance, increases power consumption, and increases device temperature. The high contact resistance of the p-contact degrades the performance of essential functions of the semiconductor device, and shortens the life of the device. Therefore, a decrease in the contact resistance of the p-contact is sought.
Ideally, a p-contact or n-contact is a metal-to-semiconductor contact with a contact resistance that is sufficiently small that the contact resistance can be ignored compared with the bulk resistance of the semiconductor in series with the contact. The performance of a resistive contact can be characterized by its specific contact resistance, Rc. The specific contact resistance Rc is the reciprocal of the value partial differentiation of the drive current I with respect to the drive voltage V when the drive voltage of the semiconductor device is V=0. Since the contact resistance Rc depends exponentially on the height .phi..sub.B of the Schottky barrier between the metal electrode and the semiconductor, the contact resistance can be reduced by reducing the barrier height .phi..sub.B. Moreover, in a region in which the concentration N of impurities in the semiconductor is sufficiently high for the tunnelling current to be dominant, the specific contact resistance Rc depends exponentially on .phi..sub.B N.sup.-1/2, so increasing the impurity concentration N is also effective to decrease the specific contact resistance.
In the device described in the above-mentioned NIKKEI ELECTRONICS article, the impurity concentration in the p-type GaN is believed to be about ten times greater than 10.sup.18 cm.sup.-3. Such high impurity concentrations in p-type GaN result in a significant drop in the activation rate of the acceptors or in an extreme degradation of crystallinity, so favorable results are not obtained.
In Nonalloyed Ohmic Contacts on GaN Using InN/GaN Short-Period Superlattice, 64 APPL. PHYS. LETT., No. 19, pp. 2557-59, (May 9, 1994), M. E. Lin et al. disclose an example of depositing electrodes composed of two layers of Ti/Al on n-type GaN and annealing at 900.degree. C. for a short time of 30 seconds to obtain a specific contact resistance of 8.times.10.sup.-6 .OMEGA..cm.sup.2. This paper also discloses an example of depositing an InN/GaN short-period superlattice (SPS) on a GaN layer and depositing a highly-doped InN layer on the SPS structure as a cap layer. A metal electrode is deposited on the cap layer. This structure provides a contact with a specific contact resistance of 8 .times.10.sup.-5 .OMEGA..cm.sup.2 without annealing. The quantum tunnel effect passing through the SPS conduction band is thought to narrow the effective band gap. FIG. 2 shows a schematic band diagram of the contact described in the article. To form the SPS, n-type InN and n-type GaN were alternately deposited in layers on an n-type GaN layer. The impurity level of the n-type GaN of the GaN layer and in the SPS was 5.times.10.sup.18 cm.sup.-3. The impurity level of the n-type InN of the cap layer and in the SPS was 1.times.10.sup.19 cm.sup.-3, so the metal-semiconductor structure whose band diagram is shown in FIG. 2 has no ability to function as a rectifier.
It is not clear whether either of the contact structures described above can be effectively used to lower the specific contact resistance of a p-contact made to p-type GaN or other p-type Group III-nitride semiconductor. Therefore, new ways of lowering the specific contact resistance of a p-contact made to p-type Group III-nitride semiconductors are being sought.
In devices based on GaAs semiconductors, In.sub.x Ga.sub.1-x As is used as a cap layer to which the p-contact is made. The band gap of In.sub.x Ga.sub.1-x As is 0.75 eV when x=0.53. This is narrower than the band gap of of GaAs, which is 1.42 eV. Lei et al., referred to above, describe this contact structure as providing a low specific contact resistance. Even if using a material with a narrower band gap than the p-type Group III-nitride semiconductor as a cap layer for the p-contact results in a decreased specific contact resistance, it is still unclear whether this contact structure can be fabricated without a complicated processing. In.sub.x Ga.sub.1-x N is a potential material for such a cap layer, but since the In content of the In.sub.x Ga.sub.1-x N has a relatively small bending effect on the band gap, In.sub.x Ga.sub.1-x N with a large In fraction must be used to narrow the band gap significantly. Diffusing In into GaN to form In.sub.x Ga.sub.1-x N with a large In fraction is difficult in itself, and additionally causes problems due to the large lattice mismatch between In.sub.x Ga.sub.1-x N and GaN.
In Electrical Characteristics and Interface Structure of Ni/Au Contact Material on a p-Type GaN, PROC. OF THE 42ND CONF. OF THE JPNS. SOC. OF APPL. PHYS., Lecture No. 30a-ZH-8, 19951 (Spring 1995)(in Japanese), Kobayashi et al. disclosed a specific contact resistance lowered to 10.sup.-2 .OMEGA..cm.sup.2. This was achieved by activating at a high temperature (800.degree. C.) GaN:Mg in which Mg impurities had been introduced into GaN. Metal electrodes of Au/Ni were vapor deposited after a hole concentration in the range of 4 to 8.times.10.sup.17 cm.sup.-3 had been reached. Finally, the structure was annealed. However, this value of specific resistance is insufficient by a number of orders of magnitude, and the demand for a lower specific contact resistance still remains.
If a p-contact with an acceptably-low specific contact resistance can be fabricated inexpensively on p-type GaN, and can provide better reliability, the possibility exists that a similar contact structure can be applied to other p-type Group III-nitride semiconductor devices and not just to p-type gallium nitride semiconductor devices.
What is needed is a p-contact between a metal electrode and a p-type Group III-nitride semiconductor that has a low specific contact resistance and that can be formed using a relatively simple method.
What is also needed is a p-contact between a metal electrode and a p-type Group III-nitride semiconductor with a low a Schottky barrier and a low specific contact resistance and that can be fabricated using a relatively simple method.
What is also needed is a p-type Group III-nitride semiconductor device that has a p-contact with a low Schottky barrier and a low specific contact resistance.
Finally, what is needed is a method of fabricating a p-type Group III-nitride semiconductor device that can easily be made using some of the same manufacturing processes as other elements of the device.